Currently when we build WiredTiger with

-DCMAKE_BUILD_TYPE=TSan
-DCMAKE_TOOLCHAIN_FILE=../cmake/toolchains/mongodbtoolchain_stable_gcc.cmake 

we get the error

CMake Error at cmake/configs/modes.cmake:199 (message):
  Build type 'TSan' not available.
Call Stack (most recent call first):
  CMakeLists.txt:14 (include) 

If we run cmake with --log-level=VERBOSE this is because our gcc version is missing required C flags:

Skipping build mode definition due to unavailable C flags: TSan 

however this is not clear from a default run of CMake.

This ticket is to improve the error reporting of CMake. If the define_build_mode function fails to add our specified build mode Cmake should report this clearly in the failure message.